1. Field of the Invention
The present invention relates to a belt device used in an image forming apparatus, such as a copier, a printer, and a facsimile machine, and more particularly, to a belt device including a belt used as an intermediate transfer unit and an image forming apparatus including the belt device.
2. Description of the Related Art
In a typical image forming apparatus, such as a printer, a copier, a facsimile machine, and a printing press, an endless belt is used as a latent-image carrier, an image transfer medium, or a conveying member for conveying a recording medium such as a recording sheet.
Such an endless belt is generally supported by a plurality of rollers, and is driven to move in a predetermined direction by any one of the rollers.
Namely, the endless belt moves by having frictional contact with the rollers. While the belt is moving, it may happen that the belt meanders due to a friction acting in an axial direction of the rollers, a tilt of an axis line, or the like. In this case, the meandering of the belt means that the belt moves by being biased toward either one side of the belt in a width direction, which is parallel to the axial direction of the rollers.
When a full-color image is formed by a color image forming apparatus employing an intermediate transfer method, a plurality of different color toner images are sequentially transferred onto an intermediate transfer belt in a superimposed manner. However, if the intermediate transfer belt meanders, the toner images cannot be transferred onto a predetermined transfer position on the intermediate transfer belt, and thus an error such as a color registration error occurs in a formed image.
To prevent such a meandering of a belt, various methods have been developed. For example, in a conventional technology disclosed in Japanese Patent No. 3523503, a blocking member is integrally fixed to a rear surface of a belt at a position close to an end face of a roller. If the belt meanders, the blocking member collides with the end face of the roller. Therefore, it is possible to prevent the belt from being biased toward either one side in the width direction.
Furthermore, for example, in conventional technologies disclosed in Japanese Patent Application Laid-open No. H05-204199, Japanese Patent Application Laid-open No. 2004-226746, and Japanese Patent Application Laid-open No. H11-161055, in addition to the blocking member as described above, a belt is configured to be pressed so that the belt can be prevented from being biased toward either one side in the width direction.
Moreover, for example, in conventional technologies disclosed in Japanese Patent No. 3223771 and Japanese Patent Application Laid-open No. H05-134486, instead of such a blocking member, a flange is provided on both end faces of a roller in the axial direction so as to prevent a meandering of a belt.
Furthermore, it has been recognized that, in an image forming apparatus employing an intermediate transfer method, a range where a color registration error occurs in a transfer process can be reduced by an increase of a tension of an intermediate transfer belt. This is because a frictional force generated between a drive roller and the intermediate transfer belt increases with the increase of the belt tension. Therefore, in conventional technologies, to ensure a sufficient belt tension, such a tension roller that a spring is attached to both ends thereof is pressed against a belt.
In this method, however, with the increase of the belt tension, it may easily cause such problems that the belt has a crack on its end portion or the belt is curled up. In addition, a lateral deviation of the belt tension may occur due to a tolerance of a spring tension of each of the springs.
For example, it can be said that there is a difference in a belt tension between an intermediate transfer belt tensed by a tension roller that a spring having a spring tension of an upper limit tolerance is attached to both ends thereof and an intermediate transfer belt tensed by a tension roller that a spring having a spring tension of a lower limit tolerance is attached to both ends thereof. Furthermore, when a spring having a spring tension of an upper limit tolerance is attached to one end of a tension roller and a spring having a spring tension of a lower limit tolerance is attached to the other end of the tension roller, a lateral deviation in a belt tension occurs. A spring has a spring tension of a higher tolerance with an increase of spring pressure in general. Therefore, even when it is possible to ensure a high belt tension with preventing an occurrence of a crack on an end portion of the belt or curling of the belt, for example, by the application of pressure/depressure, there is still a possibility of an occurrence of a considerable lateral deviation in a belt tension because the belt tension is obtained by the spring pressure. Moreover, if a displacement of the belt in the width direction or a bias (a meandering) of the belt occurs while the belt is moving, it may cause damage to the end portion of the belt, and thus a belt device including the belt or an image forming apparatus including the belt device may fail to ensure a sufficient durability.
To solve the above problems, for example, in a transfer device disclosed in Japanese Patent No. 3523503, a transfer belt is an endless belt supported by rollers, and moves to a transfer position where a toner image formed on an image carrier is to be transferred in accordance with rotation of the rollers. In the transfer device, as described above, to prevent a meandering of the transfer belt, the blocking member is provided on both end portions of the rear surface of the transfer belt so that the blocking members are guided by the end faces of the rollers. In addition, an electric resistance of each of the blocking members is set up to be identical to that of a transfer unit. Furthermore, in an endless belt type carrying device disclosed in Japanese Patent No. 3223771, a plurality of roller members other than a drive roller are configured to be tiltable. Therefore, a belt walk can be corrected in such a manner that a contact pressure of each of the roller members with respect to an endless belt is biased toward either one side in an axial direction of the roller members by adjusting a tilting angle of each of the roller members. Moreover, in an endless belt disclosed in Japanese Patent No. 3837246, a guide member is provided on the endless belt. When the endless belt meanders, the guide member is struck on an end face of a belt supporting roller or a flange provided on the end face of the belt supporting roller. Therefore, it is possible to prevent the endless belt from meandering.
However, in the transfer device disclosed in Japanese Patent No. 3523503, it is necessary to install the blocking members on the rear surface of the belt having difficulty in being handled, so that the productivity is decreased. In addition, it is necessary to form a guide groove for each of the blocking members, which are relatively thick, on each of supporting rollers that respectively support the belt. Therefore, a diameter of each of the supporting rollers is increased, and thus a size of the entire apparatus is also increased. In the endless belt type carrying device disclosed in Japanese Patent No. 3223771, it is necessary to provide a belt-meandering detecting unit, a roller-tilt correcting unit, and the like. Therefore, a configuration of the device becomes complicated, and a production cost and a size of the entire apparatus are increased. In the endless belt disclosed in Japanese Patent No. 3837246, an excess bias force in the width direction of the endless belt is generated due to, for example, a positional tolerance among the supporting rollers, a deviation of a belt tension in a direction of an axis line of the endless belt, and a deviation of a friction coefficient among the supporting rollers. Therefore, it is difficult to keep preventing a buckling distortion of an end portion of the belt stably for a long time.
Furthermore, in such a configuration that the flange is provided on both end faces of the roller in the axial direction to prevent a meandering of the belt, although it is possible to avoid an impact generated when the blocking member provided on the rear surface of the belt collides with the end face of the roller, there are other problems as follows.
If there is a positional tolerance among the rollers supporting the belt or a deviation of a tension of the belt in the axial direction of the rollers due to a tilt of an axis line of each of the rollers, an excess bias force is exerted on the belt in the width direction. Furthermore, an excess bias force is generated when each of the rollers differs in a coefficient of friction in the axial direction.
When the belt is excessively biased in the width direction, and struck on the flange, the belt is buckled. If such a state that the belt is buckled is continued for a long time, the durability of the belt is impaired. It is possible to use a high stiffness belt in consideration of the durability of the belt; however, a cost increases in this case.
Moreover, the above problems can be solved by an installation of a pressing member. With only the pressing member, it is possible to downsize the apparatus and reduce a cost without decreasing the productivity. However, in this case, it may cause such a problem that a toner leaking from an end portion of a cleaning unit for cleaning an intermediate transfer belt is deposited on the pressing member, and the deposited toner is scattered inside the apparatus. Consequently, an energization error may arise because inside the apparatus including the intermediate transfer belt is stained with the scattered toner, or the image forming apparatus may fail to control an image density or an image forming process because a light receiving unit of a toner-mark sensor is stained with the scattered toner.